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Building the Survivor: the BFO

I must have a thing for oscillators, since it’s the first thing I’ve tackled in this project. The previous installment covered the variable frequency oscillator (VFO). This installment covers the other oscillator in the circuit, the beat frequency oscillator (BFO). In the Survivor, the same BFO is used on both receive and transmit.

When receiving, the receive mixer (U2) mixes the whatever’s coming in from the antenna with the VFO frequency (5 – 5.3 MHz). The output of the receive mixer is a bunch of frequencies: VFO + input, VFO – input, and all sorts of other combinations. Here we’re interested in the sum of the input frequency and the VFO. When we tune the receiver, we adjust the VFO frequency so that it combines with the frequency of interest coming from the antenna such that the sum of the frequencies is 9 MHz. Then the whole works gets sent through the 9 MHz crystal ladder filter made up by X1-X5 and capacitors C32, C33, C35, and C36 to filter out the signals on unwanted frequencies. Here’s where the BFO comes in. The product detector (U1) mixes the signal from the BFO and the 9 MHz input and outputs the difference, which is the incoming signal converted to audio frequencies.

On transmit, a similar process occurs. The microphone audio input is mixed with the BFO in U2 to convert it to a 9 MHz modulated signal, sent through the crystal ladder filter, and then downconverted to 3.75 – 4 MHz in U1 by mixing it with the VFO.

You might have noticed that the BFO is used by U1 and the VFO by U2 on receive, and vice versa on transmit. The designer cleverly included an analog switch (U4) that switches the two signals back and forth, controlled by the microcontroller (U5).

The BFO design itself seems fairly straightforward. It’s a Colpitts crystal oscillator. On the schematic, it resides immediately to the left of U4. At first glance, it appeared to me to be connected directly to U2, but I was fooled by its close proximity to U2 in the drawing. I will confess that I’m not able to explain the functioning of every part in this oscillator, but in general it behaves like any oscillator, with a combination of inductance, capacitance, and feedback provided by Q17 acting as an amplifier. X6 acts to more or less force the resonance to be at 9 MHz, although trimmer capacitor CT1 can be used to pull the crystal a little off its frequency for fine adjustment purposes. (Anyone who has a better, more detailed explanation of how this particular circuit functions, please feel free to leave a comment at the end of the post!)

To build the BFO circuit, install the following parts:

R36, R46, R48

C54, C55, C61, C71, CT1 (CT1 is polarized, so make sure you align it with the outline on the board)

L2 (recall from a previous section that this is labeled as L6 on the schematic, and its correct value is 8.2 uH)

Q17 (make sure you position this part in alignment with the outline on the board)

X6

Here’s the board with the parts installed:

The board after installation of the BFOClose-up of the BFO section.

To test, you’ll need the digital dial again. Connect the digital dial’s power leads to your power supply or battery, and connect its signal lead to the pad labeled BFO (immediately to the right of R37). Connect power to the board and short the ON/OFF pads to “turn on the switch.” The digital dial should read something very close to 9 MHz (mine read 8.9995). For fun, try adjusting CT1 to see how much of an effect it has on the frequency.